Catalyst systems for lowering epoxy resin cure temperatures

ABSTRACT

New structural adhesives are disclosed in which an epoxy resin is cured with dicyandiamide. The curing of the epoxy resin by dicyandiamide is accelerated by the addition of appropriate amounts of organic compounds containing certain active groups such as SCN, NH, NH2, SH, N, or CN, in combination with azo, hydroxyl, H:N, or sulfur groups.

United States Patent [1 1 Lombardi et al.

[451 Sept. 2, 1975 CATALYST SYSTEMS FOR LOWERING EPOXY RESIN CURE TEMPERATURES [75] Inventors: Andrew D. Lombardi, Pittsburgh;

Thomas W. Smeal, Delmont, both of [73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

[22] Filed: Apr. 2, 1973 [21] Appl. N0.: 347,270

[52] US. Cl. 260/47 EN; 260/2 N; 260/37 EP; 260/59; 260/9l.3 VA

[51] Int. Cl. C08G 30/14 [58] Field of Search 260/47 EN, 47 EC, 2 N, 260/2 EC, 830, 59, 91.3 VA; 252/182 [56] References Cited UNITED STATES PATENTS 8/1958 Wear 260/2 3,397,156 8/1968 Lopez et a]. 260/47 3,554,967 l/1971 Uelzmann 260/47 3,562,215 2/1971 Moore 260/830 Primary ExaminerMelvin Goldstein Assistant ExaminerT. Pertilla Attorney, Agent, or Firm-William L. Krayer 5 7 ABSTRACT 7 Claims, No Drawings CATALYST SYSTEMS FOR LOWERING EPOXY RESIN CURE TEMPERATURES BACKGROUND OF THE INVENTION and (4) cure times are shortened significantly. This invention is different from the prior practices described above in that it combines high strength and high heat resistance with moderate cure temperatures and short cure times. All of the prior practices are deficient in at least one of these qualities. As an example, the only approach described above that we know to be in commer- It has been known prior to the present invention to 5 cial use is the use of latent curatives composed of comcure epoxy resins with dicyandiamide. 'See, for examl organic polysalte S h an dh i h b P NOS-i 3,297,635, 3,420,794, 3,530,093 tested and has been found to develop low adhesive and Netherlands Patent Application No. 6,609,550- strengths, particularly peel strength and especially at It e also been known in the P21st to use Various types temperatures below 67F and above 250F. In addiof accelerators for other curing agents. See, for examtion h resistance to h aging was poor, l h h h ple: Schroeder, US. Pat. No. 2,831,830; Bressler, U.S. cure temperature and time were good.

Mika, 3,091,595; The epoxy ether resins suitable for use in the compo- Rainer, 3,189,646; Fulmef, sitions of the presentinvention include the epoxy poly- Venik, 2 3 Newey, ethers of polyhydric phenols obtained by reacting a Greene, polyhydric phenol with a halogen-containing epoxide Hickner, 3,415,902; and in an alkaline medium. Polyhydric phenols that can be Perse, US. 3,472,610; and South African used for this purpose include, among others, resorcinol, catechol, hydroquinone, methylresorcinol, polynuclear Dicyandiamide has not found wide use as a curing phenols, Such as 2,2 bis(4 hydroxyphenyl) propane agent because until the present invention a suitable ac- 2O (Bis phenol A); 2,2 biS (4 hydroxyphenyl) butane; celerator has not been found to reduce the temperature 4 4 h bi 4 h d h l)- of decomposition of dicyandiamide which is so high ethane; 2 2 (4 hydroxyphenyl) pemane; and as to cause Problems of metal wafpage, diseol' 1,5-dihydroxynaphthalene and the like. The halogenoration, excessive curing times due to the necessity of Containing epoxides may be lifi d by 1 h 2 heating up the assembly, and excessive residual stress 3 epoxypropane (epichlorohydrin); 2 in the bond due to the wide temperature difference beepoxybutane; 3 2 3 2 tween Curing and Service of the p epoxyoctane; and the like. Preferred polyepoxides of Excellent structural adhesives have been made using this type are the glycidyl polyethers of dihydric phenols dicyandiamide (dicy) as acurative for epoxy resin; produced by the method from dihydric phenols and however, the temperatures required to react to dicy are epiehlorohydrin The monomer products of this type too high for many applications, or the time at temperamay be represented by the general formula: ture is too long.

It is known that the curing of epoxy resin by dicy is O accelerated by amines, amides, quaternary compounds, substituted ureas and melamines, zinc and cadmium CH2-CHCH2OROCH2CH--CH, salts of amines, and other amine salts. However, these all have serious shortcomings because they adversely where R represents divalent hydrocarbon radical of the affect strength and durability, or have only a slight acdihydric phenol, having up to 20 or more carbon celerating effect. Latent curatives other than dicy, such atoms, usually not more than 16. The polymeric prodas (l) dihydrazides, (2) extra-coordinate siliconate ucts will generally not be a single simple molecule but salts, (3) Lewis acid salts and (4) complex organic will be a complex of glycidyl polyethers of the general polysalts have also been used. The first two of these are formula:

CH2CHCH2(ORO-CH2-CHOH-4CH2),,O-ROCH,CH-CH;,

too slow and require too high a temperature; the third wherein R is a divalent hydrocarbon radical of the dihygenerally produces adhesives with mediocre adhesive dric phenol and n is an integer of the series 0, l, 2, 3, strengths; and the adhesives that employ the fourth apetc. While for any single molecule of the polether n is proach have adversely affected properties, especially so an integer, the fact that the obtained polyether is a mixpeel strength and resistance to aging at elevated temture of compounds causes the determined value for n peratures, although cure is accomplished quickly and to be an average which is not necessarily zero or a at relatively low temperature, The technologies of miwhole number. The polyethers may in some cases concroencapsulation and molecular sieves have never been tain a very small amount of material with one or both adapted to structural epoxy adhesives, and the use of of the terminal glycidyl radicals in hydrated form. frozen epoxies is limited by practical aspects. The aforedescribed glycidyl polyethers of the dihydric phenols may be prepared by reacting the required SUMMARY OF THE INVENTION proportions of the dihydric phenol and the epichloro- This invention utilizes a group of accelerators, for the hydrin in an alkaline medium. The desired alkalinity is reaction of dicy with epoxy, that has not previously obtained by adding basic substances, such as sodium or been recognized as being able to accelerate this reacpotassium hydroxide. The reaction is preferably action. its advantages are: (1) high adhesive strengths are complished at temperatures within the range of from in l g resistance to heat g ng is 50 C to 150 C. The heating is continued for several tained, (3) cure temperatures are lowered significantly, hours to effect the reaction and the product is then washed free of salt and base.

These epoxy resins are avaiable in several forms varying from a viscous'liquid to a solid resin. Especially suit- Typical of the-epoxy resinswhichmay be employed.

are the epichlorohydrin-bis-phenol type sold under the trademarks Epon Resins (Shell Chemical Corporation, Gen Epoxy (General Mills). ,DER Resins (Dow Chemical Company),Araldite (Ciba), ERL Resins? (Bakelite Corporation), Epi-Rez (Jones Dabney), and Epiphenl (The Borden Company).

Another group of polyepoxides that can, be used comprises the glycidyl ethers of novolak resins, which resins are obtained by condensing an aldehyde with a phenol ,under acid conditions. Typical members of this class'are the epoxy resins. formed by condensing formaldehyde with phenol or cresol and reacting the novolak resin thus obtained with epichlorohydrin.

Other polyepoxides include the polyepoxy. polyethers comprising ethers of epoxy alcohols and polyhydric alcohols such as obtained by reacting, preferably in the presence of an acid-acting compound such as hydrofluoric acid, polyhydric alcohols with epichlorohydrin or dichlorohydrins and then dehydrochlorinating the resulting product in the presence of an alkaline component. Examples of polyhydric alcohols that may be used for. this purpose include, ,.among others, 1,2,6- hexanetriol, 1,5-pentanediol, butylene glycol, glycerol, sorbitol, mannitol, pentaerythritol, polyallyl alcohol, polyvinyl alcohol, trimethylolpropane, bis (4- hydroxycyclohexyl). dimethylmethane, 1,4- dimethylolbenzene, and the like. Particularly preferred members of this group comprise the glycidyl polyethe rs of aliphatic polyhydric alcohols containing from 2 to carbon atoms and having from 2 to 6 hydroxyl groups and more, preferably the alkane polyols containing from 2 to 8 carbon atoms and having from 2 to 6 hydroxyl groups. Such products preferably have an epoxy equivalency greater than 1.0, and still more preferably between 1 .1 and 4 and a molecular weight between 300 and 1000.

The present invention consists of adding to a compounded dicyepoxy resin composition from 0.1 to 10.0

percent, and preferably 1.0 to 3.0 percent, of an accelerator, said accelerator being an organic compound having 2 to 12 carbon atoms and containing one or more SCN, NH, NH SH, N, or CN groups in combina- EXAMPLE 1 A reduction in gel time of a dicy epoxy adhesive was achieved by use of 2.5 percent of the indicated accelerators. The adhesive formulation was: epoxy resin (diglycidyl ether of bisphenol A having epoxy equivalent weight of 182 to 195) 49.08%, dicy 3.83%, flexibilizer 1.64%, aluminum powder 40.90%, gellant 4.10%, adhesion promoter -O.4l%, and antioxidants 0.04%. The following procedure was used to measure gel time. One-gram aliquots of adhesive were put on aluminum foil and placed in a forced draft oven operating at 300 F. for a specific period of time. Each aliquot was removed from the oven after a specific time period and immediately probed with a stainless steel rod. If the adhesive was soft, another aliquot of the same composition was placed in the oven for a longer time. If the adhesive was hard and could not be penetrated with the 5 rod, another aliquot of the same composition was placed in the oven for a shorter time. In this manner, the period of time required for the adhesive to become hard was determined very closely and recorded in the following Table l as gel time at 300 F. in minutes.

Table 1 Minutes to gel Accelerator at 300F.

2O to 30 It may be noted that the compounds listed which do not meet the requirements of this invention for active groups do not accelerate the gel of the adhesive.

EXAMPLE 2 (SEE TABLE I1) Adhesive. strengths were achieved with the unaccelerated adhesive of Example 1 and a cure cycle of 60 minutes at 350 F. The designated properties were determined by standard ASTM methods, more particularly Dl002-64 (tensile shear strength), D903-49 (peel strength), and Dl78l-6OT (climbing drum peel strength),

EXAMPLE 3 (SEE TABLE II AND III) Results show adhesive strengths achieved when 2.5 percent dimethyldithiocarbamic acid dimethylammonium salt was added to the adhesive of Example 1 and the resulting adhesive was cured for 60 minutes at 300 F. At all testing temperatures, except for the highest (300 F.), the adhesive strengths were only moderately lower than those of Example 2. However, it was found that the adhesive of Example 1 with no accelerator did not cure in 60 minutes at 300 F.

EXAMPLE 4 (SEE TABLE II) Results show adhesive strengths achieved when the adhesive of Example 3 was cured for minutes at 250 F. Although the adhesive strengths were lower than those obtained in Example 3, they were quite high at all tested temperatures below 300 F.

EXAMPLE 5 (SEE TABLE ll) Results show adhesive strengths achieved when the adhesive of Example 3 was cured for 60 minutes at 225 F. Although the cure temperature was 125 F. lower than that used in Example 2, lap-shear bond strength was adversely affected only at 300 F., the highest test temperature.

EXAMPLE 6 (SEE TABLE II) Results show adhesive strengths achieved when the adhesive of Example 3 was cured for 90 minutes at 200 F. Excellent lap-shear strength was achieved despite the low temperature, demonstrating the great ability of this compound to catalyze the decomposition of dicy.

EXAMPLE 7 (SEE TABLE ll) EXAMPLE 8 (SEE TABLE) Results show adhesive strengths achieved with a commercial one-component adhesive utilizing a complex organic polysalt curative when it was cured at 300 F. for 60 minutes.

novel combination of dicyandiamide and accelerator therefor used as a curing agent for epoxy resins. The resulting resin exhibits high adhesive strength, good resistance to heat aging and relatively low cure temperatures and short cure times. Our invention also includes a method of accelerating the cure of epoxy resins using dicyandiamide as a curing agent.

We claim:

1. A composition comprising a 1,2-epoxy ether resin having an epoxy equivalency greater than one, from about one to about twenty percent dicyandiamide, and from about one to about ten percent of an accelerator selected from the group consisting of dimethyl dithiocarbamic acid dimethylammonium salt, bis(dimethylthiocarbamyl) sulfide, tetramethylthiuram disulfide, and tetraethylthiuram disulfide.

2. Composition of claim 1 in which the epoxy resin is a polyglycidyl ether of a polyhydric phenol.

3. Composition of claim 1 in which the epoxy resin 0 is a polyglycidyl ether of bisphenol A having an epoxy Table II Example 2 3 4 5 6 7 8 Lap shear at 67 F. 7400 6750 6050 6250 5950 7100 3750 77 F. 6950 6750 5150 6000 5200 6600 5450 180 F. 6100 4600 5200 4900 5900 300 F. 3750 600 500 1400 500 1000 I600 I80-Degrce peel at 67 F. 63 63 23 4 5 57 3 77 F. 55 48 36 22 6 47 28 300 F. 30 20 7 l0 6 19 28 Climbing-drum peel 47 23 4 5 58 EX PL 9 equivalent weight of 170 to 1200.

The effect of aging specimens bonded with adhesives of Examples 3 8 at 400 F. for 7 days is shown in Table III.

It can be seen that the commercial adhesive of Example 8 was much more affected by heat aging that was the adhesive of this invention as in Example 3.

EXAMPLE 10 1.7 percent tetramethylthiruam disulfide was added to the adhesive of Example 1, and the resulting adhesive was cured at 350 F. for 10 minutes. A lap-shear bond strength of 4900 psi was achieved at 80 F.

Mixtures of several of the accelerators may produce improved efiects and faster cures and yet lengthen effective shelf life by permitting the use of low concentrations of the more reactive accelerators.

Thus, it may be seen that our invention provides a 4. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent, and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula [[(CH NCS] S]. 5. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent; and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula [[(Cl-l NCS] S 6. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent; and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula 7. A method of accelerating the rate of curing of a 1,2-epoxy ether resin having an epoxy equivalency greater than one with dicyandiamide comprising adding to the epoxy resin-dicyandiamide composition from 0.1 to about 10 percent of an accelerator selected from the group consisting of dimethyldithiocarbamic acid dimethylammonium salt, bis(dimethylthiocarbamyl) sulfide, tetramethylthiuram disulfide, and tetraethylthiuram disulfide.

UNITED STATES PATENT OFFICE CETIFICATE 0F CORECTION Patent No. 3,903,048 Dated September 2, 1975 InV n Andrew D. Lombardi et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 41, after "composition" insert -(containing from about one to about twenty percent dicyandiamide)--.

Column 5, Table II, all entries in last line, "Climbing-drum peel (ippi)",

should be moved one column to the right.

Table III, in first heading "Example 3" should read --Adhesive of Example 3--.

Signed an fiealed this first ay 0? June 1976 [SEAL] Atresr:

RUTH c. MASON c. MARSHALL DANN Alluring Officer Commissioner ofParemx and Trademarks 

1. A COMPOSITION COMPRISING A 1,2-EPOXY ETHER RESIN HAVING AN EPOXY EQUIVALENCY GREATER THAN ONE, FROM ABOUT ONE TO ABOUT TWENTY PERCENT DICYANDIAMIDE, AND FROM ABOUT ONE TO ABOUT TEN PERCENT OF AN ACCELERATOR SELECTED FROM THE GROUP CONSISTING OF DIMETHYL DITHIOCARBAMIC ACID DIMETHYLAMMONIUM SALT, BIS(DIMETHYLTHIOCARBAMY) SULFIDE, TETRAMETHYLTHIURAM DISULFIDE, AND TETRAETHYLTHIURAM DISULFIDE.
 2. Composition of claim 1 in which the epoxy resin is a polyglycidyl ether of a polyhydric phenol.
 3. Composition of claim 1 in which the epoxy resin is a polyglycidyl ether of bisphenol A having an epoxy equivalent weight of 170 to
 1200. 4. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent, and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula (((CH3)2NCS)2S).
 5. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent; and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula (((CH3)2NCS)2S2).
 6. A curable composition of matter consisting essentially of a. a polyepoxide whose molecule contains on an average more than one 1,2-epoxide groups; b. dicyandiamide as curing agent; and c. as curing accelerator 0.1 to about 10 percent of a composition of the formula ((CH3)2NCSSNH2(CH3)2).
 7. A method of accelerating the rate of curing of a 1,2-epoxy ether resin having an epoxy equivalency Greater than one with dicyandiamide comprising adding to the epoxy resin-dicyandiamide composition from 0.1 to about 10 percent of an accelerator selected from the group consisting of dimethyldithiocarbamic acid dimethylammonium salt, bis(dimethylthiocarbamyl) sulfide, tetramethylthiuram disulfide, and tetraethylthiuram disulfide. 